An instance with the same name is already installed on this computer. To proceed with SQL Server setup, provide a unique instance name.

You may get below error while installing SQL Server.

An instance with the same name is already installed on this computer. To proceed with SQL Server setup, provide a unique instance name.

The main reason of above error is:

The named Instance, you are trying to install is already present.
The existing instance of SQL Server 2008 is a unique named instance. However, you try to install SQL Server 2005 as the default instance.
The existing instance of SQL Server 2008 is a named instance, and you use the same name when you try to install SQL Server 2005 as a named instance.

Follow below steps to solve the error (Suppose you want to create instance name SQLExpress:
Click Start menu, select Run and type regedit
Navigate to
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\
      Find “MSSQL$SQLExpress” , and delete
Go to HKEY_LOCAL_MACHINE\Software\Microsoft\Microsoft SQL Server
     Remove "SQLExpress" from the REG_MULTI_SZ value named InstalledInstance
     Delete the subhive named MSSQL.1
     Delete the subhive named SQLExpress
Go to HKEY_LOCAL_MACHINE\Software\Microsoft\Microsoft SQL Server\Instance Names\SQL
    Delete the value named  "SQLExpress"
Go to SQL Server installation drive Delete the folder %ProgramFiles%\Microsoft SQL Server\MSSQL.1
Restart the PC.

Use either Windows installer cleanup utility to manually remove all traces of MSSQL in your PC.

Re-run SQL installation

Alternatively, you can try installing SQL Server with different instance name.